1. Field of the Invention
The present invention relates to alignment metrology, and in particular to an alignment target and method of use.
2. Discussion of the Related Art
Semiconductor processing for forming integrated circuits requires a series of processing steps. These processing steps include the deposition and patterning of material layers such as insulating layers, polysilicon layers, and metal layers. The material layers are typically patterned using a photoresist layer that is patterned over the material layer using a photomask or reticle. It is important that one layer is aligned with another during processing.
Typically, the photomask has alignment targets or keys that are aligned to fiduciary marks formed in the previous layer on the substrate. However, as the integrated circuit feature sizes continue to decrease to provide increasing circuit density, it becomes increasingly difficult to measure the alignment accuracy of one masking level to the previous level. This overlay metrology problem becomes particularly difficult at feature sizes below approximately 100 nm where overlay alignment tolerances are reduced to provide reliable semiconductor devices.
FIGS. 1A and 1B show conventional overlay targets used with conventional imaging metrology methods. FIG. 1A shows a typical Box-in-Box overlay target 2. Target 2 is formed by producing an etched box 4 in a material layer 6 on a substrate. A corresponding smaller box 8 on the photomask or reticle is aligned to the larger box 4 so that the centers of the large and small boxes are aligned.
FIG. 1B shows a Bar-in-Bar overlay target 12, which is similar to target 2 shown in FIG. 1A. Target 12 is produced by etching bars 14 in a material layer 16 on a substrate. The bars 18 on the photomask are aligned to the overlay target alignment bars 14.
After the smaller box 8 or bars 18 are developed, i.e., exposed and etched, the overlay target is imaged to determine whether the photomask or reticle was properly aligned with the underlying layer. Conventionally, high magnification imaging is used to measure overlay alignment. Conventional imaging devices, unfortunately, suffer from disadvantages such as sensitivity to vibration and cost. Moreover, conventional imaging devices suffer from a trade-off between depth-of-focus and optical resolution. Additionally, edge-detection algorithms used to analyze images for the purpose of extracting overlay error are inaccurate when the imaged target is inherently low-contrast or when the target suffers from asymmetries due to wafer processing.
During processing, the substrate is moved from one location to the next so that different areas, e.g., dies, on the substrate can be exposed. The alignment system, e.g., the exposure tool, typically uses an alignment target to properly align the substrate during exposure. FIG. 2 shows a conventional alignment system 50, which includes a diffraction pattern 52 on the substrate and a second diffraction pattern 54 that is stationary, e.g., is fixed to the lens on the exposure tool. A light source 56 produces coherent light that passes through a beam splitter 58 and is incident on the diffraction pattern 52 after passing through a lens 60. The light is diffracted by diffraction pattern 52 and passes through lens 60 back to beam splitter 58. The coherent light beam from source 56 is also reflected off beam splitter 58, passes through lens 62 and is incident on diffraction pattern 54. The light diffracted by diffraction pattern 54 passes back through lens 62 to beam splitter 58. At beam splitter the light diffracted from diffraction patterns 52 and 54 is combined and the combined diffraction light is received by light detectors 64.
Alignment system 50 provides an accuracy of approximately 15 nm. One disadvantage of alignment system 50 is that coherent light is used. Thus, if the diffraction pattern 52 on the sample absorbs the particular frequency used, alignment system 50 cannot provide an accurate measurement. While multiple coherent light sources may be used to avoid this disadvantage, the use of multiple light sources adds complexity and cost.
Thus, there is a need in the semiconductor industry for an improved alignment target for metrology and alignment system.